1. Field of the Invention
This invention relates to printed circuit boards, and more specifically, to the testing of integrated passive components.
2. Description of the Related Art
As advances in computer systems and electronic systems continue, many printed circuit assemblies may be designed to operate at higher frequencies. For example, personal computers having a motherboard with a bus speed exceeding 100 MHz are typical, and this speed will almost certainly increase in the future.
One design consideration that plays a greater role in high-frequency printed circuit assemblies is parasitics. Parasitics may be defined as those electrical characteristics which may be negligible at lower frequencies, but play an increasing role as frequency increases. Parasitic characteristics that must be considered for highfrequency printed circuit assemblies include parasitic capacitances and parasitic inductances. These parasitic electrical characteristics may be affected by many different factors, such as length and width of a circuit trace, pad geometry for surface mounted components, and so on. Parasitics may become a major limitation at high frequencies in the design of printed circuit assemblies.
The use of integrated passive components may help reduce the effects of parasitics. Integrated passive components are components that are integrated directly into or onto a printed circuit board. Resistors, capacitors, and inductors may be implemented in a circuit using integrated passives. Integrated passive components may be assembled into the appropriate layer of a printed circuit board prior to lamination. Following the placement of integrated passives, additional layers of material may be added to the printed circuit board, followed by a lamination process.
In order to ensure that the proper types and values of integrated passive components are integrated into a printed circuit board, testing may be required. Testing may occur either prior to or following lamination of the printed circuit board, or both. Testing of circuit boards with integrated passive components is often accomplished by test systems having movable probes, which may be referred to as xe2x80x9cflying probexe2x80x9d testers. Flying probe testers may include several test probes. When testing a two-terminal component, such as a resistor, a flying probe tester may place a probe on each of the nodes to which terminals of the component are electrically connected. Additional probes may be placed at nearby circuit nodes in order to compensate for unwanted shunt currents, thereby enabling a more accurate reading of the component""s electrical parameters.
Testing of integrated passive components prior to lamination may present various difficulties. Prior to lamination, the various layers of a printed circuit board may be very flexible and fragile. It may not be possible to establish a good test connection with pads or circuit traces coupled to the integrated passive component under test, as a highpressure test probe contact could damage the inner layers of the printed circuit board. However, if pressure of test probe contact is insufficient, test results may be inaccurate.
Following lamination, test probes may make a high-pressure contact with a printed circuit board with minimal risk of damage. However, it may not be possible to conduct repairs to the printed circuit board following lamination. Thus if an integrated passive component is damaged or has a value that is not within specification, the printed circuit board may have to be scrapped.
The problems outlined above may in large part be solved by a system and method for testing integrated passive components in a printed circuit board. In one embodiment, testing of integrated passive components may be conducted prior to completing the final lamination of the printed circuit board. The testing may be conducted on a tester having movable test probes. The method may include connecting a first test probe to a conductive plane, which may be electrically connected to the first terminals of two or more components. The conductive plane may be a ground plane, a power plane, or a signal plane. The first test probe may remain in a fixed position throughout the testing. A second test probe may be electrically connected to the second terminal of the first component. Following the connection of the second test probe, an electrical characteristic of the first component may be measured. Following the measurement of an electrical characteristic of the first component, the second test probe may then be repositioned in order to be electrically connected to the second terminal of a second component. After repositioning and electrically connecting the second test probe to the second terminal of the second component, an electrical characteristic of the second component may be measured. This may be repeated for any number of components to be tested. The first test probe, which is electrically connected to a conductive plane to which each of the tested components is electrically connected, may remain in a fixed position throughout the testing, while the second test probe moves from component to component.
In another embodiment, testing of integrated passive components may be conducted subsequent to completing the final lamination of the printed circuit board. The testing may include electrically connecting a first test probe to a first conductive plane, such as a power plane, and a second test probe to a second conductive plane, such as a ground plane. A first terminal of each of a plurality of components may be electrically connected to either the first or second conductive planes. A second terminal of each of the plurality of components may be connected to a signal path, which may be electrically isolated from the signal paths to which the other components are electrically connected. A test head having a shorting probe may be used to electrically couple the signal path of a first component under test to either the first plane or the second plane. By shorting the signal path to one of the planes, a current path is created from the first plane, through the first component under test, to the second plane. This may allow for the measurement of an electrical characteristic of the first component under test. Following the measurement of an electrical characteristic for the first component, the test head having a shorting probe may be moved in order to electrically couple the signal path of a second component under test to either the first plane or second plane. This may allow for the measurement of an electrical characteristic of the second component under test. This may be repeated for any number or components under test. The first test probe and second test probe may remain in a fixed position during the test, while the test head moves the shorting probe in order to create the necessary current path for the component under test.
The testing of the integrated passive components may be performed on a test system having movable probes and/or test heads. The test system may include at least a first probe and a second probe, each of which are capable of being repositioned in order to be electrically connected to various locations on the printed circuit board. The test system may further include a movable test head to which a shorting probe may be attached. Software in the test system may cause the tester to perform the testing for integrated passive components, as described above, prior to or following the final lamination of the printed circuit board.
Electrical characteristics that may be measured include, but are not limited to, resistance, capacitance, and inductance. The types of components for which measurements may be performed may include, but are not limited to, resistors, inductors, and capacitors. Other embodiments are possible and contemplated wherein the measurement of secondary electrical characteristics may be performed. Such secondary electrical characteristics may include the equivalent series resistance of a capacitor or the impedance of an inductor.